Glow discharge lamps for use in spectroscopic analyzers

ABSTRACT

A glow discharge lamp provided with an anode and a cathode which comprises a first cylindrical cup member and a second cylindrical cup member having a smaller outer diameter than that of the first cylindrical cup member and being inserted vaportightly into the first cylindrical cup member with a space therebetween for accommodating a lump of a low-melting-point metal, the hollows of the two cup members are communicated with each other by means of a through hole having a predetermined diameter such that an amount of vapor of the low-melting-point metal may be controlled in the hollow of the second cylindrical cup member, whereby the lump is heated by electron discharge caused between the anode and the cathode and effectively vaporized so that the desirable amount of the vapor flows into the second cylindrical cup member.

United States Patent l l 1 I I Inventors Kuuo Yasuda;

fliroslli Olugaki, both of Katsuta-shi, Japan Appl No, 824,971

Filed May [5, i969 Patented July 27, I971 Assignee Hitachi. Ltd.

Tokyo, Japan Priority May 15, 1968 Japan 43132213 GLOW DISCHARGE LAMPS FOR USE IN SPECTROSCOPIC ANALYZERS 7 Claims, 8 Drawing Figs.

U.S.Cl 313/209, 3l3/2l0,3l3/2ll,3l3/2l7,3l3/346 Int. Cl H01] 61/04 FieldotSearch 313/209,

iil

[ 56] References Cited UNITED STATES PATENTS 3,264,5ll 8/1966 Yamasakinnnw .l 313/209 3,474,280 10/1969 Vollmerr,........, in 313/346X Primary ExaminerRaymond F. Hossfeld AtrorneyCraig, Antonelli and Hill ABSTRACT: A glow discharge lamp provided with an anode and a cathode which comprises a first cylindrical cup member and a second cylindrical cup member having a smaller outer diameter than that of the first cylindrical cup member and being inserted vaportightly into the first cylindrical cup member with a space therebetween for accommodating a lump of a low-melting-point metal, the hollows of the two cup members are communicated with each other by means of a through hole having a predetermined diameter such that an amount of vapor of the low-melting-point metal may be controlled in the hollow of the second cylindrical cup member, whereby the lump is heated by electron discharge caused between the anode and the cathode and effectively vaporized so that the desirable amount of the vapor flows into the second cylindrical cup member.

ATENTED JUL27 IQTI SHEEI 1 OF 4 ""lprannuraraa r I I I v 1 I 1 I I I r I gua y SuDA Jud HIRosHI Dana M,Mvw

ATTORNEYS PATEN1EDJUL2'II9TI 3,596,127

SHEET 2 F 4 FIG. 5

'50 a E D. 0 it O LU 004357 2 |00- O (D E E 0: U1 50 4 IO 3O 40 5O 6O CURRENT(mA) I5OL' A FIG. 6 E

DC ITOV lb 50 4b 50 6o CURRENT(mA) MEMO OPERATiNG DC VOLTAGE DROP or KAI U0 YA s 0 DA AND Hmouu u OKAGAN Cyan AntuneLLL, St UJOJIL ATTORNEYS PATENTEDJUL2TI97I 3.596.127

saw u UF 4 FIG. 8

RELATIVE INTENSI m'vmons IO IO CLRRENT (FHA) KAIUO YASvDH AND HIROSHI OKHEAKI BY CYQKJ, Antonelli, Sheuxwt k H1 ATTORNEYS GLOW DISCHARGE LAMPS FOR USE IN SPECTROSCOPIC ANALYZERS BACKGROU ND OF THE INVENTION This invention relates to an improved glow discharge lamp for use in a spectroscopic analyzer and more particularly, to an improved glow discharge lamp provided with a hollow cathode which is very suitable for analysis or inspection of low-melting-point metals such as sodium, potassium, lithium, indium, lead or the like.

The principle of generation of Fraunhoffer lines has been applied for analysis of metallic elements in atomic absorption analysis. When a metallic vapor is in thermal equilibrium at a certain temperature, a portion of the metal atoms are in an excited state by heat energy, while most of the metal atoms are in the ground state. When light of a wavelength corresponding to the energy level of the particular atom irradiates the metal lic vapor of the sample, the light is absorbed by particular atoms. The amount of light absorbed in this case has a definite relationship to the amount of metal atoms in the vapor, and quantitative analysis of the metal atoms contained in an unknown sample can be effected by measurement of the rate of light absorption.

ln practical atomic absorption analysis, resonance lines are mainly used as the irradiation light. However, in general, the wavelength width of resonance lines is affected by minute con struction of the spectrum, natural width, Doppler width, pressure efl'ect, resonance effect, Stark effect, self-absorption, etc. A high-precision analysis can be expected if resonance lines of narrow wavelength width are employed. Among the aboveenumerated factors affecting the wavelength width, the most important factors are Doppler width and self-absorption. Since Doppler width is caused by the thermal movement olluminescent atoms, a hollow cathode discharge lamp is mainly used, which can be operated at low temperatures in the range near the abnormal glow discharge.

A conventional glow discharge lamp having a hollow cathode heretofore generally used comprises a cathode of hollow cylindrical shape constituted by elements to be analyzed, but in case of low-melting-point metals, particularly alkali metals, they are so readily oxidized and ionized that it is dif ficult to make the hollow cathode of such metals.

ln order to solve the above problem, an attempt was made to use hollow cathodes made of alloys containing alkali metals, or a discharge tube confining an alkali metal vapor and having a hollow cathode and an anode of an electrocond uctive material of a high melting point. In the former, however, since there are limits to the containable amount of alkali metals, the strength of light and a service-life of the device proved unsatisfactory. In the latter, undesirable discharge was found to take place between the anode and the outer peripheral portions of the hollow cathode.

By the above reasons, discharge lamps having thermionic emission means have been necessarily used in the case of lowmelting-point metals, particularly, alkali metals. ln this case, another electrical supply source is required to energize the thermionic emission means than that for energizing the conventional discharge of the hollow cathode, resulting in a complicated construction of the lamp. Moreover, in thermionic emission where discharge lamp self-absorption of resonance lines is great, good analytical precision cannot be obtained.

ln order to eliminate the above problem, we further made such glow discharge lamps as described in U.S. Pat. No. 3,286,119, wherein the hollow cathode for accommodating a low-melting-point metal, and the inner peripheral portion of the hollow cathode is made of a refractory electroconductive substance, which permits diffusion and permeation therethrough of the low-meltingpoint metal. In this hollow cathode the inner peripheral portion is made by powder metallurgy in order to provide a porous body capable of diffusion and permeation of the vapor of the low-melting-point metal. Since the amount of the vapor which flows into the inner peripheral portion may be determined by the porosity of the porous body, the porosity must be made proper. In practical cases, however, the control of the porosity is very difficult. In addition, even if the porosity of the porous body is proper, the proper porosity and continuity of the interstices of the sintered porous body is apt to deteriorate when the body is worked or machined so as to form a desirable hollow cathode shape. Therefore, the diffusion and permeation may be suppressed considerably. Probably the porosity of the sintered body may be increased in sintered body so as to improve the diffusion and permeation of the vapor of the lowmelting point metal. In practice, however, it is also difficult to adjust and control properly the porosity of all manufactures. As a result, these manufactured cathodes are not always capable of maintaining a spectral light emission of the low-melting-point metals with a desirable strength during a desirable service life thereof. And nonuniform distribution of porosity and the insufficient continuity of the interstices in the sintered body necessarily cause the partial diffusion and partial permeation so that all the inner surface of the hollow cathode cannot contribute to spectral light emission and flicker of the light cmission point is caused in the hollow cathode. Furthermore, the above-mentioned hollow cathodes produced by powder metallurgy tend to be expensive because a number of production steps cannot be involved in the manufacturing.

From the above reasons, the hollow cathodes which can emit spectral light of the low-melting-point metal, such as sodium, potassium, lithium or the like and which are satisfactory from the practical point of view, have not been provided yet. Because the atomic resonance absorption spectrometry has been widely employed in the fields of medical science, biological science, and biochemistry, the requirement has increased that the glow discharge lamps be capable of emitting spectral light of alkali metals. Therefore, the solution of the above described problems is very important in the art.

SUMMARY OF THE INVENTION Accordingly, one of the objects of the present invention is to provide an improved discharge lamp for use in spectroscopic analyzers.

Another object of the invention is to provide a glow discharge lamp capable of emitting atomic-absorption spectral-light of low-melting-point metals having a sufficient intensity for the service life thereof A further object of the invention is to provide improved means capable of obtaining with a sufficient strength the desired spectral light of the low-melting-point metal.

A further object of the invention is to provide an improved glow discharge lamp having such a structure as to be easily manufactured.

A still further object of the invention is to provide an improved hollow cathode larnp in which glow discharge is maintained only between the anode and the inner surface of the hollow-cathode and a high efficiency of the glow discharge can be attained.

Basically, the present invention provides a glow discharge hollow cathode lamp in which the cathode structure has a first hollow to be subjected to glow discharge and to emit a spectral light of the low-melting-point metal and a second hollow able to accommodate a low-melting-point metal, partition means provided between the first and second hollows and having a through hole of a predetermined diameter to communicate the first and second hollows with each other, and a piece of the low-melting-point metal accommodated in the second hol low, whereby during heating of the cathode owing to glow discharge a predetermined amount of the vapor may be supplied from the first hollow to the second hollow.

The present invention may be applied to any kind of lowmelting-point metals. Practical examples of them may be Li (m.p. 179 C.), Na (97 C.), K (87 C.), Cd (320.9 C.), In (156.6 C.), T1(303.6 C. Sn (231.9 C.), Pb (327.3 C.), Bi (271.4C.), or Se (217.4 C.).

Further objects and advantages of the invention will become apparent as the following description proceeds and features of the novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, reference may be had to the accompanying drawings, in which:

FIG. 1 is a side elevational view in section showing a glow discharge lamp embodying the invention;

FIG. 2 is a side elevational view in section showing in detail the cathode structure of the invention shown in FIG. I;

FIG. 3 is a side elevational view in section showing in detail another construction of electrodes embodying the invention;

FIG. 4 is a perspective exploded view, partially broken away of respective members of the cathode shown in FIG. 3;

FIG, 5 and FIG. 6 show characteristic curves of glow discharge lamps according to the invention, plotting the operating DC voltage (V) against discharge current (mA) for the glow discharge lamp, wherein cathodes contain sodium and potassium respectively; and

FIG. 7 and FIG. 8 are graphical representations showing, respectively, the intensity of spectral lines versus discharge current (mA) characteristic curves for the same glow discharge lamps.

DETAlLED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show an embodiment of a glow discharge lamp of the present invention. A cylindrical envelope 10 made ofa suitable material, such as glass, has an enlarged portion 33 and a smaller tubular portion 29, wherein the smaller tubular portion 29 is sealed off at one end 21 by a closed tubular member l2 which is made ola suitable material such as quartz efliciently transmissive of light of wavelengths produced by the lamp. The portion 33 is sealed off by a button stem header 34 having a suitable tipped off exhaust tube 36 in a manner well known in the art.

Supported by the header 34 and disposed within the portion 33, there is positioned a cathode 2 which is constituted by an inner cylindrical cup member 16 having a hole [3 in the bottom wall 16 and an extending portion 30 with an opening 31, and an outer cylindrical cup member 14 wherein a glow discharge portion is formed in the hollow of the inner cylindrical cup member and an accommodating space 24 for a lump 1] or piece of a low-melting-point metal is formed between the outer face of the inner cylindrical cup member l6 and the inner face of the outer cylindrical cup member 14, and the hollow of the cylindrical cup member 16 communicates with the accommodating space 24 through the hole 13. The inner cylindrical cup member 16 is screwed vaportightly into the outer cylindrical cup member 14 at the threaded portion.

As an entrance for the vapor of the low-melting-point metal accommodated in the cup member 14, the through hole 13 has a suitable diameter determined by taking into consideration the gas pressure in the cylindrical envelope 10, the diameter of opening 31 and the kind of low-melting-point metal to be accommodated and other technical points of view, and the through hole 13 has such a construction as not to present any resistance against flow of the vapor of the low-melting-point metal. The dimension of the accommodating space 24 may be chosen in consideration of the amount of low-melting-point material to be accommodated, the structures of the two cylindrical cup members 14 and I6, and operating conditions of the glow discharge lamp. These cylindrical cup members 14 and [6 are made of a suitable electroconductive and considerably high-melting-point material, preferably, nickel, aluminum, copper or an alloy thereof, and it may be also desirable that the materials of the cylindrical cupmembers l4 and 16 have a low sputtering rate,

The outer cylindrical cup member 14 is covered with a capshaped case member 25 made of a suitable electroconductive high-melting-point material such as nickel or the like, whereby the case member 25 is supported by a cathode lead 23 in the cylindrical envelope [0.

The anode 22 which may be formed in a ring shape in general is connected electroconductively with a plurality of support rods 26 by welding or other means. At least one of the support rods 26 is of an electroconductive material, for example, the same material as that ofthe anode. A pair ofelongated insulating sleeves 28 surround the support rods 26 between the lower surface ofa first insulating disk 18 and the top of the button stem header 34. The first insulating disk 18 is of any suitable material such as mica, wherein the central aperture has a diameter that permits projection of the extending cylindrical portion 30 therethrough. The ring-shaped anode 22 is isolated from the first insulating disk 18 by cylindrical insulating spacer pieces 20 and a second insulating disk 18'. The

second insulating disk 18' has also a central aperture 19 which i has a diameter which may be intermediate the inner diameter and the outer diameter of the extending portion 30 but which is smaller than the inner diameter of portion 30. The insulating disks l8 and 18 have respective holes for the support rods 26 to pass therethrough. An insulating base member 15 has holes for the projecting connecting rods 17 and a cylindrical member 27 for supporting the envelope 10.

In operation, a direct current potential (21 nonreversible current potential) of an appropriate value is applied from a source (not illustrated) between the cathode 2 and the ring shaped anode 22 in a manner that the cathode is negative with respect to the anode 22. This potential causes the gas confined in the envelope to ionize and a glow discharge to take place between these two electrodes. As will be described, the discharge is concentrated between the interior surface of the inner cylindrical cup member 16 and the anode 22. The bombardment olthis surface by ions (He*, Ar Nefl etc) causes the cathode to sputter and emit radiation in accordance with the material of which the lump 11 is composed. This emitted radiation of spectral light is transmitted along the axis of the envelope 10 and passes through the window 12 so that it may be utilized in accordance with methods well known in the art to analyze or inspect a desired sample material.

Further in detail, mainly due to the glow discharge the cathode is heated to such a temperature that the low-melting point metal is melted and vaporized, successively. Then the vapor of the low-melting-point metal passes through the hole l3 into the inner cylindrical cup member 16. The drifting vapor in the inner cylindrical cup member 16 thus entered is struck by ionized gas atoms confined in the envelope [0 so that metal atoms of the vapor are energized to a higher electron energy level than that of the ground state The energized atoms can emit light of a specific wavelength on the charac teristic line spectrum inherent with the metal atoms when the excited electrons return to the ground state. As well known, this light having a specific wavelength is utilized in spectro scopic analyzers. The amount of the inert gas to be confined in the envelope is chosen such that the gas pressure may be about i to H) mm. Hg, especially about 3 to 6 mm. Hg so as to obtain a good spectral light beam of high stability. When the pressure is less than l mm. Hg, the amount of the inert gas atoms that contribute to the glow discharge is too small. On the contrary, when the pressure is more than 10 mm. Hg, the width of the spectral line or characteristic line spectra is so large that the analytical precision of the spectrometry will be deteriorated.

The aforementioned glow discharge may be caused only between the inner surface of the cylindrical cup member 16 and the ring-shaped anode 22 because there is provided insulating disks 1! and 18' and moreover the outer face of the outer cylindrical cup member 14 is covered with the cylindri cal case member 25 made of a high-melting-point material. As described above, the outer cylindrical cup member 14 and the cylindrical extending portion 30, support rods 26 and the inner cylindrical cup member 16 are msde of the metallic material of a considerably higher melting point, such as tungsten, tantalum, nickel, molybdenum, than that of the lowmelting-point metal. Therefore, any inconveniences such as deformation and melting thereof are avoided sufficiently.

Materials of at least the inner surface of the inner cylindrical cup member 16 may be selected from the electroconductive materials being intimate with the vapor of the low-melting-point metals. lf alkali metals are selected as the lovwmelting-point metals, aluminum or its alloy may be preferably emplayed as the material of the inner cylindrical cup member 16.

Another preferred embodiment is illustrated in FIG. 3, wherein only the structure of the cathode 3 is different from that of FIG. 1 and FIG. 2. In FIG. 3, the glow discharge lamp includes a cylindrical envelope 40, a ring-shaped anode 43 connected with support rods 44, a first insulating disk 45 and a second insulating disk 45, elongated insulating sleeves 42 surrounding the support rods 44. Electrically conducting means 56 connect with terminal 59 of the cathode 3. The first insulating disc 45 and the second insulating disc 45' are separated by an insulating ring 52 from each other, and the second insulating disk 45' is also separated by another insulating disk 52' from the anode 43. In this embodiment, the two insulating rings 52 and 52' are preferably of steatite, taking into consideration the mechanical strength and economy thereof. However, the surface areas thereof may preferably be made small as possible as long as it does not deteriorate the mechanical strength of the insulating disks, in order to avoid the problem wherein owing to deposition of the low-meltingpoint metal on the surface of the insulating disks an electrical insulating property thereof tends to be deteriorated and undesirable discharge tends to take place between the anode and the insulating rings 52 and 52'. For the above reason, it is preferable that the insulating rings 52 and 52' made of steatite are provided as disk-shaped rings having the smallest permissible surface, as illustrated in FIG. 3, instead of the thin cylindrical spacer pieces 20. According to investigation and consideration with respect to the cause of the occurrence of undesirable discharge between the anode and the surfaces of the insulating rings 52 and 52', it may be believed that the steatite has an afiinity for the vapor of the low-melting-point metal owing to activity of the vaporized metal. Deposition of the low-melting-point metal causes reduction of metal oxide in the material of steatite so that the surface of the steatite tends to form a metallic surface and to cause undesirable discharge between the anode and the steatite surfaces. if the material of the insulating rings have a small affinity for the vapor, it may not be necessary to make such structure as shown in FIG. 3.

As described above, the insulating disks 45 and 45' are of mica because of a small affinity for the vapor of the low-melting-point metal. in other words, mica is difiicult to be reduced by the deposited low-melting-point metal and therefore, the insulating ring 52 and 52' are preferably of mica or the like. The peripheries of the insulating disks are made to extend near the inner surface of the cylindrical envelope to avoid undesirable discharge between the anode and the outer face of the cathode 3.

The cathode 3 of which the structure will be made more ap parent from the illustration in FIG. 4, wherein the cathode comprises a cylindrical cap member 47 having a flange portion and an opening 58 forming an irradiating port for spectral light beam, an inner cylindrical cup member 50 provided with a plurality of through holes 60 in the bottom thereof, and an outer cylindrical cup member 48 for accommodating the piece of lump 53 of the low-melting-point metal in a space formed between the inner cylindrical cup member and the outer cylindrical cup member 48, and the outer cylindrical cup member 48 is covered with a cylindrical case member 46 of high melting point material having a small sputtering rate, such as nickel. The cylindrical case member 46 has a terminal 59 to be connected with a cathode lead 56. The inner cylindrical cup member 50 is inserted in the outer cylindrical cup member 48 to form a space portion (an accommodating portion) 41 separated from the hollow of the inner cylindrical cup member by the bottom wall thereof, and then the cap member 47 is inserted into the outer cylindrical cup member 48 to seal off the space vaportightly other than the through holes at the screw portion. The piece or lump 53 of the low-melting-point metal is accommodated and vaporized in the space portion 41.

In this embodiment, though the glow discharge is caused in the same manner as described in connection with the other embodiment shown in FIGS. 1 and 2, this construction of the cathode 3 brings better results than the other. One of the advantages over the cathode shown in FIG. I and 2 is that the spill out of the melted low-melting-point metal from the opening 55 may sufficiently be prevented by a barrier portion of the cap member 47, since the diameter of the opening 55 is made smaller than that of the hollow of the inner cylindrical cup member 50. Another advantage is that supply of the vapor of the low-melting-point metal into the inner cylindrical member may be made more properly because a plurality of small holes 60 (through holes having predetermined diameters, respectively) provided in the bottom wall of the inner cylindrical cup member 50 are arranged to supply the vapor of the low-melting-point metal in such directions to cover substantially all the inner surface of the inner cylindrical cup member wherein each of the small holes 60 is formed to constitute an angle of about 45 with the axis of the cathode. As a result, the glow discharge in the inner cylindrical cup member may be made very effective. it is preferable that the axes of the through holes have such an inclination that the energized gas atoms cannot arrive directly at the space portion through the holes. And the holes have such a construction that the peripheries of the holes in the outer and inner face of the bottom wall do overlap each other in the horizontal direction (the longitudinal direction) of the inner cylindrical cup member so as to avoid direct bombardment of the space portion 4i by energized gas atoms which may pass through the holes of otherwise oriented.

According to one embodiment of the invention, the gas pressure in the cylindrical envelope 40 was made about 3 to 6 mm. Hg, the diameter of the opening 55 was made about 3 mm., the inner diameter of the inner cylindrical cup member 48 was made about 5 mm, and the diameter of each of the small holes 60 was made about 0.5 to 1.5 mm. Since the diameter of the plurality of the holes decides the amount of the vapor of the Iow-melting-point metal to be supplied to the inner cylindrical cup member, and also affects the properties of the flow discharge lamp, such as wavelength width thereof, they have to be formed precisely to have a predetermined diameter. in other words, the holes 60 have such a cort struction that the vapor entering into the inner cylindrical cup member from the outer cylindrical cup member may contact with substantially all the inner surface of the inner cylindrical cup member.

An excess amount of vapor in 'the inner cylindrical rup member may cause the self-absorption of the spectral li ht. According to the above construction of the cathode, desirable results may be obtained by controlling the amplitude of the direct current to be about 10 to 30 mA, especially about 15 to 25 mA when the low-melting-point metal is alkali metals.

As understood from the foregoing description, the present invention has eliminated serious defects of conventional hollow cathode lamps and has made it possible to manufacture practical ones for an atomic absorption analysis of alkali metals and other low-msltlng-polnt metals. In addition, the hollow cathode discharge lamps of the invention are easy to manufacture and have a high operational stability and long service life. Further, in atomic absorption analysers employing a hollow cathode lamp of the invention, no additional electrical source is necessary for heating and vaporizing the lowmelting-point metal, resulting in a simple construction and {structure of a cost. Furthermore, it is a very important adtive discharge lamp is considerably smaller than that of the vantage that the self-absorption of spectral light of the invenconventional discharge lamp having thermionic emission means, because the amount of the drifting vapor is made proper by controlling the diameter of the holes according to the invention. On the contrary, in the conventional discharge lamp it is very difficult to decide precisely the above factors. The above points will be apparent from the following description in reference to the accompanying drawings FIGS. to 8.

in FIGS. 5 and 6 characteristic curves of discharge current versus operating DC voltage drop of glow discharge lamps for sodium and potassium are indicated. respectively. As to the glow discharge device for sodium, though the curve has a center-raised form, this characteristic may not be a disadvantage in practical use at all. Reduction of the operating DC voltage drop, observed in the range of the current beyond about 25 mA, may be due to an excess amount of the vapor in the inner cylindrical cup member. That is, an excess amount of the vapor may cause a decrease in the operating DC voltage drop by virtue of an increment of glow discharge. However, when such glow discharge is undesirably caused by the excess vapor which drifts by the anode, it was observed that a rather large self-absorption of the spectral light is emitted from the inner cylindrical cup member. Therefore, in case of a glow discharge lamp of sodium, the current to be applied to the electrodes may be made less than about 30 mA. On the other hand, reduction of the operating DC voltage drop was observed in the range of the current less than about [0 mA. However, in the above case, the intensity of the spectral light beam may not be sufficient to utilize the glow discharge lamp in spectroscopic analyzers. As will be understood by taking into consideration the form of the curve shown in FIG. 5, the preferable current flow between the electrodes may be in a range ofabout to mA.

In case of potassium, however, such large reduction of the operating DC voltage drop as observed in the case of sodium was not observed, as shown in H6. 6. And the characteristic curve has relatively flat form. When considering the intensity and self-absorption of the spectral light beam, it has been found that the desirable current is from about 15 to mA.

From FIGS. 7 and 8, it will be easily determined that the intensities of the characteristic spectral lines according to the invention are very high, that is, the intensities are about 10 times greater than those of a conventional glow discharge lamp, such as a glow discharge lamp including a cathode comprising a sintered body of low-meltingpoint metal. In FIGS. 7 and 8, bend-points of the characteristic curves are observed in the range of the current beyond about 25 to 30 mA. These result may be caused by self-absorption of the spectral light. Large current beyond about 30 or mA may also cause the increase in analytical precision owing to Doppler width which is caused by thermal movement. From the above, a current flow in the electrodes of less than about 25 mA may be preferable. On the other hand, in case of a current of about less than 10 MA, the intensity of the spectral light beam is too small to utilize effectively the spectral light beam in a spectroscopic analyzer. However, when another construction of the cathode or operating condition is employed, the most preferable value of current may be determined by taking into consideration the above-mentioned results and other technical points of view.

By the way, absorbed gas, such as oxygen, hydrogen, nitrogen and the like, in the constituting members of the cathode is believed to cause deterioration of analytical preci sion. Therefore, to reduce the absorbed gas to an extremely small amount is very important and manufacture of the cathode should be done, taking the above fact into consideration. It goes without saying that for reduction of the absorbed gas, the solid integral metallic material for the cathode constituting members such as the cap member, the inner cylindrical cup member and the outer cylindrical cup member is more preferable than the porous body manufactured by powder metallurgy. If a high vacuum in an inert or reduction atmosphere is employed, however, the sintering porous body or solid body may be of course used as the members, efl'ectively. According to investigation, it was found that the solid integral metallic material of aluminum copper, nickel or its alloy are suitable material as the members. lron material containing carbon may not be suitable material for the member unless the carbon is removed. Carbon contained in the material may provide carbon oxide during operation of the lamp. To make small the amount of the absorbed gas and to remove such in convenience as flicker of the glow discharge, the inner surfaces of the opening 55 of the cap member and the inner cylindrical cup member are made as smooth as possible by working or machining.

While there have been shown and described what are at present considered to be preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the artv It is not intended, therefore, that the invention be limited to the specific arrangements shown and described, but that it encompass all such modifications as fall within the true spirit and scope of the invention.

What we claim is:

l. A glow discharge lamp for use in a spectroscopic analyzer comprising an envelope confining an inert gaseous atmosphere and having a portion transmissive to spectral light; a ring-shaped anode and a cathode disposed in spaced relationship within said envelope; a pair of electrically conducting means respectively connected with the anode and the cathode for maintaining an electron discharge therebetween', and refractory insulating means provided between the anode and the cathode and having an aperture through which a beam of spectral light may pass; wherein the cathode comprises a first cup member, a second cup member having a smaller diameter than said first cup member and defining a first chamber for radiating a beam of spectral light, at least the bottom wall thereof being inserted into the first cup member to form a second chamber for accommodating a piece of a low-meltingpoint metal to be vaporized and being provided with a hole for communicating the first and second chambers with each other, the hole having such a diameter as to permit a predetermined amount of flow of vapor of the low-melting-point metal, and a cylindrical case member of a low-sputter rate and a high-melting-point material covering the outer face of the first cup member, the refractory insulating means being constituted to isolate the cathode from the anode by at least a pair of insulating discs which have a small affinity for the vapor and are separated from each other by means of a refractory insulating ring having an inner diameter at least as wide as the opening of the insulating discs.

2. In a glow discharge lamp for use in a spectroscopic analyzer comprising an envelope confining an inert gaseous atmosphere and having a portion transmissive to spectral light; a ringshaped anode and a cathode disposed in spaced rela tionship within the envelope for maintaining an electron discharge therebetween; a pair of electrically conducting means respectively connected with the anode and the cathode; and refractory insulating means provided between the anode and the cathode and having an aperture through which a beam of spectral light may pass; the improvement being characterized in that the cathode comprises a first cup member, a second cup member having a smaller diameter than said first cup member and being inserted into said first cup member in order to define a first chamber for accommodating a piece of a low-melting-point metal to be vaporized therein and a second chamber for radiating the spectral light of the low-melting-point metal, said first and second chambers being in communication by means of a hole permitting a predetermined amount of flow of vapor of the low-meltingpoint metal, and a cylindrical cap member provided with an opening having a smaller diameter than the hollow of the second cup member for radiating the beam of the spectral light therethrough, and the outer face of the first cup member being covered with a case member of a small-sputter-rate and a high-melting-point metal.

3. A glow discharge lamp according to claim 2, wherein the refractory insulating means comprises a first insulating disc having a central aperture of a diameter at least as wide as the opening of the cap member and a second insulating disc having a central aperture of a diameter at least as wide as the aperture of the first insulating disc, and the first and second insulating discs and the cathode being separated from the anode by an annularshaped refractory member having a central aperture of a diameter at leas as wide as the apertures of said first and second insulating discs.

4. A glow discharge lamp according to claim 2 wherein said hole between said first and second chambers has such a configuration that the vapor of the low-melting-point metal may be supplied to substantially all the inner surface of the second cup member.

5. A glow discharge lamp according to claim 2, wherein a plurality of small holes having an inclination to the horizontal axis of the cathode is provided between said first and second chambers so as to provide a resistance against the glow discharge between the anode and the first chamber.

6. In a glow discharge lamp for use in a spectroscopic analyzer comprising an envelope confining an inert gaseous atmosphere and having a portion transmissive to spectral light; a ring-shaped anode and a cathode disposed in spaced relationlhip within the envelope; electrically conducting means respectively connected with the anode and the cathode; and means for electrically insulating the anode from the cathode; the improvement being characterized in that said cathode comprises a first chamber for accommodating a piece of low melting-point metal to be vaporized therein, and a second chamber provided with an opening to emit the spectral light beam for causing a glow discharge by virtue of electron discharge, said first and second chambers being in communication with each other by means of a plurality of holes which are so constituted to allow flowing a predetermined amount of vapor of the low-melting-point metal, each hole being inclined with respect to the horizontal axis of the cathode, whereby the vapor may be supplied to substantially all the inner surface of said opening of the second chamber.

7. A hollow cathode lamp according to claim 6, wherein said first and second chambers are constituted by means of a partition member provided with said holes. 

1. A glow discharge lamp for use in a spectroscopic analyzer comprising an envelope confining an inert gaseous atmosphere and having a portion transmissive to spectral light; a ring-shaped anode and a cathode disposed in spaced relationship within said envelope; a pair of electrically conducting means respectively connected with the anode and the cathode for maintaining an electron discharge therebetween; and refractory insulating means provided between the anode and the cathode and having an aperture through which a beam of spectral light may pass; wherein the cathode comprises a first cup member, a second cup member having a smaller diameter than said first cup member and defining a first chamber for radiating a beam of spectral light, at least the bottom wall thereof being inserted into the first cup member to form a second chamber for accommodating a piece of a lowmelting-point metal to be vaporized and being provided with a hole for communicating the first and second chambers with each other, the hole having such a diameter as to permit a predetermined amount of flow of vapor of the low-melting-point metal, and a cylindrical case member of a low-sputter-rate and a high-melting-point material covering the outer face of the first cup member, the refractory insulating means being constituted to isolate the cathode from the anode by at least a pair of insulating discs which have a small affinity for the vapor and are separated from each other by means of a refractory insulating ring having an inner diameter at least as wide as the opening of the insulating discs.
 2. In a glow discharge lamp for use in a spectroscopic analyzer comprising an envelope confining an inert gaseous atmosphere and having a portion transmissive to spectral light; a ring-shaped anode and a cathode disposed in spaced relationship within the envelope for maintaining an electron discharge therebetween; a pair of electrically conducting means respectively connected with the anode and the cathode; and refractory insulating means provided between the anode and the cathode and having an aperture through which a beam of spectral light may pass; the improvement being characterized in that the cathode comprises a first cup member, a second cup member having a smaller diameter than said first cup member and being inserted into said first cup member in order to define a first chamber for accommodating a piece of a low-melting-point metal to be vaporized therein and a second chamber for radiating the spectral light of the low-melting-point metal, said first and second chambers being in communication by means of a hole permitting a predetermined amount of flow of vapor of the low-melting-point metal, and a cylindrical cap member provided with an opening having a smaller diameter than the hollow of the second cup member for radiating the beam of the spectral light therethrough, and the outer face of the first cup member being covered with a case member of a small-sputter-rate and a high-melting-point metal.
 3. A glow discharge lamp according to claim 2, wherein the refractory insulating means comprises a first insulating disc having a central aperture of a diameter at least as wide as the opening of the cap member and a second insulating disc having a central aperture of a diameter at least as wide as the aperture of the first insulating disc, and the first and second insulating discs and the cathode being separated from the anode by an annular-shaped refractory member having a central aperture of a diameter at leas as wide as the apertures of said first and second insulating discs.
 4. A glow discharge lamp according to claim 2 wherein said hole between said first and second chambers has such a configuration that the vapor of the low-melting-point metal may be supplied to substantially all the inner surface of the second cup member.
 5. A glow discharge lamp according to claim 2, wherein a plurality of small holes having an inclination to the Horizontal axis of the cathode is provided between said first and second chambers so as to provide a resistance against the glow discharge between the anode and the first chamber.
 6. In a glow discharge lamp for use in a spectroscopic analyzer comprising an envelope confining an inert gaseous atmosphere and having a portion transmissive to spectral light; a ring-shaped anode and a cathode disposed in spaced relationship within the envelope; electrically conducting means respectively connected with the anode and the cathode; and means for electrically insulating the anode from the cathode; the improvement being characterized in that said cathode comprises a first chamber for accommodating a piece of low-melting-point metal to be vaporized therein, and a second chamber provided with an opening to emit the spectral light beam for causing a glow discharge by virtue of electron discharge, said first and second chambers being in communication with each other by means of a plurality of holes which are so constituted to allow flowing a predetermined amount of vapor of the low-melting-point metal, each hole being inclined with respect to the horizontal axis of the cathode, whereby the vapor may be supplied to substantially all the inner surface of said opening of the second chamber.
 7. A hollow cathode lamp according to claim 6, wherein said first and second chambers are constituted by means of a partition member provided with said holes. 